Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

3. Acids and Bases

pKa

Organic Chemistry

3. Acids and Bases

pKa

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4:44 minutes

Problem 42b

Textbook Question

Peroxyacetic acid (pK_a = 8.2) is a much weaker acid than acetic acid (pK_a = 4.74). Explain why peroxyacetic acid is a weaker acid than acetic acid.

Verified step by step guidance

Understand the concept of pKa: The pKa value is a measure of the strength of an acid. A lower pKa value indicates a stronger acid, as it implies the acid more readily donates protons (H+ ions).

Compare the pKa values: Peroxyacetic acid has a pKa of 8.2, while acetic acid has a pKa of 4.74. This indicates that peroxyacetic acid is a weaker acid because it has a higher pKa value.

Analyze the molecular structure: Peroxyacetic acid contains a peroxy group (-OOH) instead of the hydroxyl group (-OH) found in acetic acid. The presence of the extra oxygen in the peroxy group affects the acid's ability to donate a proton.

Consider the stability of the conjugate base: The conjugate base of peroxyacetic acid is less stable than that of acetic acid. The additional oxygen in the peroxy group can lead to less effective resonance stabilization of the conjugate base, making it less favorable for the acid to donate a proton.

Evaluate the electronegativity and inductive effects: The presence of the extra oxygen in peroxyacetic acid can also lead to different inductive effects, which can influence the acid's strength. The electron-withdrawing effect of the additional oxygen may not be as strong as the resonance stabilization in acetic acid, contributing to the weaker acidity of peroxyacetic acid.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pKa and Acid Strength

The pKa value is a measure of the acidity of a compound, indicating how easily it donates a proton. A lower pKa value corresponds to a stronger acid, as it dissociates more readily in solution. In this context, acetic acid with a pKa of 4.74 is a stronger acid than peroxyacetic acid, which has a pKa of 8.2, meaning it is less willing to donate a proton.

Structure and Stability of Conjugate Base

The stability of the conjugate base formed after an acid donates a proton is crucial in determining acid strength. Acetic acid forms a more stable acetate ion due to resonance stabilization, whereas peroxyacetic acid forms a less stable conjugate base because the additional oxygen disrupts resonance and increases electron density, making it less stable and thus a weaker acid.

Electronegativity and Inductive Effect

Electronegativity and the inductive effect influence acid strength by affecting electron distribution. In peroxyacetic acid, the presence of an extra oxygen atom increases electron density near the acidic proton, reducing its ability to dissociate. This inductive effect, where electronegative atoms pull electron density, makes peroxyacetic acid a weaker acid compared to acetic acid.

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Related Practice

Textbook Question

For the bases shown, draw the conjugate acid and identify a pK_a value from Table 4.5 that would help you accurately estimate its stability.

(e)

Textbook Question

Hydrogen gas (H₂) has a relatively high pK_a value. Is it a stable or unstable acid? Do you expect it to participate in acid–base reactions?

Textbook Question

Arrange each group of compounds in order of increasing basicity.

c. sodium benzoate, sodium ethoxide, and sodium phenoxide

d. pyridine, sodium ethoxide, and sodium acetate

Textbook Question

Hydrogen peroxide (HOOH) has a pK_a of 11.6, making it roughly 10,000 times as strong an acid as water (pK_a = 15.7). Explain why H₂O₂ is a stronger acid than H₂O.